1. Field of the Invention
The present invention relates to a lithographic method.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g., including part of, one or several dies) on a substrate (e.g., a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the beam in a given direction (the “scanning” direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
When a substrate has been exposed to radiation across its entire surface, it is provided with a plurality of exposed fields, or exposed images. It is often desirable to determine the positions of these fields with a high degree of accuracy. This is because a target portion of a substrate may be exposed to a radiation beam on a number of different occasions in order to, for example, add different layers to a device. In other words, fields may be applied to the substrate on top of one another (i.e., overlaid). If fields for a given target portion are not applied accurately on top of one another, a given device formed from such fields may not function as intended, or may not function at all. For example, it may well be that a field electrically connects one functional layer of the resultant device to another functional layer of the resultant device. If the successively applied layers are not applied accurately on top of one another, the electrical connections may not be made to the correct parts of the functional layers, meaning that the device may not function properly or at all. There is often some tolerance allowed with regard to how accurately successively applied fields can be positioned on top of one another while still resulting in an acceptable device, or other structure. This is often referred to as an overlay requirement.
As will be appreciated from the previous paragraph, as well as knowing the positions of fields previously applied to a substrate, it is also desirable to know accurately a subsequent field will be applied to the substrate. For this reason, in between the application of overlaid fields, positional properties of the substrate and/or the fields may be determined. For example, before the exposure of a set of overlaying fields, alignment marks on the substrate may be used to determine if the substrate has changed in size or in orientation between successive exposures. The positions of individual fields are not measured in this example, the assumption being that the lithographic apparatus is accurate enough to consistently apply fields with a sufficient degree of accuracy. However, this is not always the case, meaning that successively applied fields may not be sufficiently well aligned on top of one another. In another example, the positions of all of the fields previously applied to the substrate on or in a single layer may be accurately determined using alignment apparatus in the lithographic apparatus. Such determination is undertaken in order to try to ensure that all subsequently aligned fields are accurately aligned with previously applied fields. However, the determining of the position of each and every field of a substrate using alignment apparatus in the lithographic apparatus may be time consuming, and may reduce throughput.